Conventionally, it has been prevailed as a steering apparatus an electric power steering apparatus supplying a steering assist force to a steering mechanism by driving an electric motor according to a steering torque steered by a driver who steers a driving wheel as a steering device.
In the electric power steering apparatus of this kind, with the increase in size of a vehicle in which the apparatus is mounted, a trend toward a higher output of the electric power steering apparatus has been progressed, which accelerates necessity of a larger current as a motor torque increases.
As shown above, with the higher output of the power steering apparatus, a steering torque necessary during manual steering will increase under conditions where the electric power steering apparatus is stopped. This creates difficulty in the steering.
Traditionally, in the event an error of the steering torque sensor or the like occurs, the electric power steering apparatus is stopped its operation to make sure of safety. In this situation, the steering torque necessary for manual steering becomes too large making steering difficult. Even when the error of the steering torque sensor occurs, there is a need for continuous generation of a steering assist force by drive controlling the electric motor.
For this reason, when the error of the steering torque sensor occurs, it is known so far that an electric power steering apparatus in which a steering torque is estimated by a steering torque estimating unit based on a vehicle speed signal and a steering angle signal, and the drive control of an electric motor is effected based on an estimated steering torque (e.g. see Patent Document 1).
In the prior art disclosed in Patent Document 1, however, since the steering torque is estimated based on the vehicle speed signal and the steering angle signal, and the drive control of the electric motor is effected based on the estimated steering torque, it cannot properly recognize a steering state, such as driving, without holding a handle by a driver, using the estimated steering torque. This leads to a steering state, against the driver's will, where the steering wheel will be freely turned, thereby providing a driver with an uncomfortable feeling. Besides, there has been an unsolved problem since the steering torque is estimated without regarding a road surface condition, the steering torque cannot be estimated properly, once a state of not being considered in the torque estimation model, such as a low road surface friction coefficient, or the like is reached.
To solve the above-described unsolved problem, there is known an electric power steering apparatus (e.g. see Patent Document 2). In the conventional example described in Patent document 2, by taking account of a reaction force of the road surface actually produced from the road surface using a wheel rotation speed, that is a self-aligning torque, a moderate steering assist force is generated. By calculating the self-aligning torque from a wheel rotation speed and a motor angle derived from a motor rotation angle sensor, apart from the self-aligning estimation, an error estimation of the self-aligning torque is prevented by comparing a self-aligning torque estimation value and a self-aligning torque calculating value. The estimation of a driving wheel slip using the wheel rotation speed prevents a self steering during a driving wheel slippage. Limiting a control output according to the motor angular velocity derived from the vehicle speed and the motor rotation angle sensor continues a steering assist control while securely preventing the self steering or a control error output.